1. Field of the Invention
The present invention relates to a print head for printing on recording paper, and to a driver device for driving such a print head. More particularly, the present invention relates to a thermal print head for performing printing by a method based on thermal sensitivity, thermal transfer (including dye sublimation), or ink jetting, and to a driver device for driving such a print head.
2. Description of Related Art
A printing apparatus such as a facsimile machine or printer typically adopts one of the following printing methods: a thermal sensitivity based method, whereby a print head is pressed against heat-sensitive paper to achieve printing on the paper; a thermal transfer based method (including a dye sublimation based method), whereby heat from a print head is applied to an ink ribbon coated with solid ink so that printing is achieved by the ink subliming and settling on recording paper; and an ink jetting based method, whereby ink is emitted by bubbles formed by application of heat thereto so that printing is achieved by the fine particles of the ink thus emitted being blown onto recording paper. A printing apparatus adopting any of these methods is provided with, as a print head with which to achieve printing, a thermal print head having, as heating elements, resistive elements arranged in a row. Such a thermal print head is provided with a driver device for driving the resistive elements so that the resistive elements, arranged in a row, release heat according to print data.
One conventional example of such a thermal print head is a recording head incorporating a driver device provided with MOS transistors for feeding electric current to and thereby driving heaters built with resistive elements (see Japanese Patent Application Laid-Open No. H10-138484). This recording head disclosed in Japanese Patent Application Laid-Open No. H10-138484 is provided with a correction circuit that is formed by the same fabrication process as the heater driving MOS transistors. The purpose of this correction circuit is to prevent variations in the current flowing through the heaters that result from, among others, process-associated variations in the characteristics of the heater driving MOS transistors and variations in wiring resistance.
As shown in FIG. 8, in a conventional thermal print head, a plurality of driver devices 100 are provided so as to drive resistive elements arranged in a row on a group-by-group basis. As shown in FIG. 9, these driver devices 100 are each provided with: a shift register 101 that stores data consisting of as many bits as the resistive elements that the driver device needs to drive; a plurality of inverters Inv that feed the data of the individual bits of the shift register 101 to MOS transistors Tr; a plurality of MOS transistors Tr that drive the resistive elements; and output terminals Out via which the drains of the MOS transistors Tr are connected to the resistive elements.
In the thermal print head configured as described above, print data that is fed on a bit-by-bit basis to the shift registers 101 of the individual driver devices 100 is serially stored therein. At this time, the driver devices 100 bring their respective shift registers 101 into a write-enable state one by one so that the print data of different groups are stored in the shift registers 101 of the different driver devices 100. The print data thus stored on a bit-by-bit basis in the shift registers 101 is then fed on a bit-by-bit basis to the inverters Inv. Here, each bit of the print data corresponds to each dot printed. That is, the number of bits contained in the print data corresponds to the number of dots printed.
At this time, the inverters Inv are fed with the same supply voltage VDD as the shift registers 101, and either this supply voltage VDD or a ground voltage is fed to the gates of the MOS transistors Tr. In a case where the shift registers 101 each store n-bit data and there are provided m driver devices 100, the driver devices 100 are each provided with n inverters Inv and n MOS transistors Tr so that, altogether, they control the driving of n×m resistive elements corresponding to n×m bits in total.
At any bits where the print data outputted from the shift registers 101 is low, the supply voltage VDD is fed through the inverters Inv to the gates of the MOS transistors Tr. This turns the MOS transistors Tr on, and thus electric current is fed via the output terminals Out to the resistive elements, which thus release heat and thereby achieve printing. By contrast, at any bits where the print data outputted from the shift registers 101 is high, the ground voltage is fed through the inverters Inv to the gates of the MOS transistors Tr. This turns the MOS transistors Tr off, and thus no electric current is fed via the output terminals Out to the resistive elements, which thus release no heat.
In the thermal print head configured as shown in FIGS. 8 and 9, a relationship as shown in FIG. 10 is observed between the voltage fed to the gates of the MOS transistors Tr provided in the driver devices 100 and the on-state resistance of the MOS transistors Tr. Assume that the MOS transistors Tr are given a gate width of Wa, Wb, or Wc (Wa>Wb>Wc). Then, in FIG. 10, the solid line represents the relationship observed when the MOS transistors Tr are given a gate width of Wa, the broken line represents the relationship observed when the MOS transistors Tr are given a gate width of Wb, and the dash-and-dot line represents the relationship observed when the MOS transistors Tr are given a gate width of Wc. As will be clearly understood from FIG. 10, the lower the voltage fed to the gates of the MOS transistors Tr, and the smaller the gate width, the higher the on-state resistance attributable to the voltage fed to the gates of the MOS transistors Tr and the greater the variations in that resistance among different MOS transistors Tr.
Conventionally, the driver devices 100 are fed with a supply voltage of 3 V to 5 V, and thus this supply voltage of 3 V to 5 V is fed to the MOS transistors Tr. Accordingly, to reduce the influence of the on-state resistance of the MOS transistors Tr, the MOS transistors Tr need to be given a gate width as great as 2,100 μm. This makes the dimension of the driver devices, which is built as a semiconductor integrated circuit device, along the shorter sides of the chip thereof as large as 1,400 μm. Moreover, the lower the voltage fed to the gates of the MOS transistors Tr, the higher the on-state resistance attributable to the gate width of the MOS transistors.
In the recording head disclosed in Japanese Patent Application Laid-Open No. H10-138484 mentioned above, the correction circuit is provided to reduce the influence of the just-mentioned on-state resistance of MOS transistors. However, the voltage fed through this correction circuit is inevitably lower than the supply voltage because of the resistance through the correction circuit. This creates the need to increase the gate width of the MOS transistors to reduce the influence of the on-state resistance. Moreover, the correction circuit needs to be formed by the same fabrication process as the MOS transistor, and therefore needs to be provided individually in each driver device. Thus, in a thermal print head provided with a plurality of driver devices, the region in which to form the correction circuit needs to be secured in each driver device. This hinders downsizing of the driver devices.